all.sh does not clean up out_of_source_build

[gilles-peskine-arm]

On mbedtls at 07912c9e3693d7ae5d62bfcbb8aef4daa9e3cafc with the framework at 53c7928, I run

tests/scripts/all.sh -k check_generated_files
tests/scripts/all.sh -k check_generated_files

The first call succeeds. The second complains:

Warning - there is an existing directory at '/tmp/mbedtls/out_of_source_build'
You can either delete this directory manually, or force the test by rerunning
the script as: ./tests/scripts/all.sh --force --out-of-source-dir /tmp/mbedtls/out_of_source_build

It looks like we're missing some a bit of cleanup code.